Normally, a controllable rectification device for an electric motor comprises an inverter, a control panel and a drive panel. The inverter comprises three switch elements connected in parallel. Each switch element comprises an upper bridge-arm switch and a lower bridge-arm switch. The control panel is used to collect peripheral sample signals, and to generate a PWM waveform according to a desired control strategy. The drive panel is used to generate a drive voltage according to the PWM waveform first, then to drive upper bridge-arm switches and lower bridge-arm switches of the three switch elements to connect or break circuits separately, thus a three-phase symmetry current is generated to drive the electric motor.
FIG. 1 shows a layout diagram of three switch elements in prior art. The three switch elements are IGBT (Insulated Gate Bipolar Transistor) 11, IGBT 13 and IGBT 15 respectively. IGBTa comprises an upper bridge-arm IGBTa′ and a lower bridge-arm IGBTa″ (a=11, 13, 15). The connection points of the upper bridge-arm switches and the lower bridge-arm switches of the three switch elements are connected to the three-phase windings U, V, W of the electric motor respectively. The drive panel is used to control the upper bridge-arm switch and the lower bridge-arm switch to be conducted as following order: IGBT11′-IGBT13″, IGBT11′-IGBT15″, IGBT13′-IGBT11″, IGBT13′-IGBT15″, IGBT15′-IGBT11″, and IGBT15′-IGBT13″, thus providing alternating currents to the three-phase windings U, V, W of the electric motor.
Currently, hybrid power vehicles generally use the engine and the electric motor as power sources respectively, so that the electric motor may have a low power. However, for an electric vehicle, the only power supplier is the electric motor, thus there is a demand for an electric motor with high power. For an electric motor with lower power, a normal IGBT may meet the requirement of the controllable rectification device. However, for an electric motor with high power, adoption of a normally used low power IGBT in the controllable rectification device may cause some limits of the torque and the power of the motor. If the low power IGBT is replaced by a high power IGBT, the cost of the controllable rectification device will increase rapidly, because the price of a high power IGBT is normally five times of a low power IGBT. And the power consumption of the high power IGBT is relatively large which may lead to waste of energy.